Acoustic wave devices using an acoustic wave are used for filters and duplexers in wireless communication devices such as mobile phone terminals. The acoustic wave devices include surface acoustic wave devices using a surface acoustic wave and bulk wave devices using a bulk wave. The surface acoustic wave device has a comb-shaped IDT (Interdigital Transducer) electrode formed on a piezoelectric substrate. The surface acoustic wave devices include Love wave devices or boundary acoustic wave devices including a dielectric film covering an IDT electrode. On the other hand, the bulk wave devices include piezoelectric thin film resonator devices including electrodes sandwiching the upper and lower surfaces of a piezoelectric film. In addition, the bulk wave devices also include Lamb wave devices using a Lamb wave.
The acoustic wave device includes a sealing portion that has an air space (space) above an electrode exciting an acoustic wave (IDT electrode formed on a piezoelectric substrate, or a pair of electrodes arranged on the upper and lower surfaces of a piezoelectric film so as to overlap each other in its thickness direction) in order to maintain its characteristics.
Wireless communication devices have been downsized, and thus modules into which acoustic wave devices are installed have been required to be downsized. There has been known that an acoustic wave device is located in a multilayer wiring board formed by stacking a wiring layer such as a metal and an insulating layer such as a resin to downsize and thin the module (e.g. Japanese Patent Application Publication No. 2006-351590). On the other hand, there has been known that moisture or an unreacted substance in a resin vaporizes and is emitted when the resin is heated (e.g. Japanese Patent Application Publication No. 2007-258776).
Moisture or the like penetrates into an air space (space) above an electrode exciting an acoustic wave even when the electrode is sealed by a sealing portion in an acoustic wave device built-in module including an acoustic wave device embedded in a multilayer wiring board formed by staking a wiring layer and an insulating layer. This erodes the electrode exciting the acoustic wave, and degrades characteristics as the acoustic wave device. In addition, when the acoustic wave device built-in module is mounted on the motherboard of an electronic device or the like, the moisture that has penetrated into the electrode exciting the acoustic wave vaporizes and expands because of application of heat in mounting, applies stress on the acoustic wave device, and may cause a crack.